Donepezil hydrochloride shows the acetylcholine esterase-inhibitory action and is useful for the treatment of all kinds of senile dementia, in particular being useful for prevention, treatment and amelioration of Alzheimer Disease. Donepezil hydrochloride is administered orally as usual and it may be placed for distribution and storage in a period of time before the administration. It may then be stored at patient's home for about one month at the maximum because of the property of the target disease. The stability of this medicinal substance (bulk pharmaceutical chemicals) against heat and humidity during the storage period is very important. A more stable medicinal substance of Donepezil hydrochloride is, therefore, desired. It is not known, however, that polymorphs of Donepezil hydrochloride exist. No sufficiently stable medicinal substance of Donepezil hydrochloride has been found.